A crankshaft in an internal combustion engine comprises main bearings which define the axis of rotation, webs which extend radially from the main bearings, and crankpins connected to the webs and arranged to describe a circle as the crankshaft rotates. The connecting rods are mounted via the big end bearings on the crankpins and while their lower ends connected to the crankpins move in a circle, their other ends, connected to the pistons, reciprocate within the cylinders.
The circular path followed by the lower end of the connecting rods determines the relative amount of time spent by the pistons in the different parts of their cycles. It is, however, desirable for thermodynamic reasons to be able to alter the time spent by the pistons in different parts of the combustion cycle. For example, it is more efficient for the combustion to take place at constant volume, and accordingly it is desirable for the cranking mechanism to permit the piston to pause near the top dead center position.
The present invention seeks to introduce greater flexibility into the control of the movement of an element, such as a piston, which drives or is driven by a crank mechanism.
According to the present invention, there is provided a crank mechanism for enabling a crankpin to follow other than a circular path centered on the cranking axis, the drawings showing a mechanism simulating a crankshaft that defines at least one main bearing section and one web extending radially from the main bearing section, a housing 20 mounted for sliding movement along the web, a crankpin 32 secured to the housing, and slidable guiding means 40 operative to cause the housing to slide radially along the web in synchronism with the rotation of the simulated crankshaft, such that the distance of the center of the crankpin from the center of the main bearing section varies as a function of the cranking angle.
In a preferred embodiment shown in FIGS. 1, 2, 3, and 4a-4d, to be described, the housing is formed with a slideway 40 extending transversely to the webs and receiving a slide block 42 which is journalled for rotation about an axis parallel to but offset from or eccentrically mounted with respect to the axis of the main bearing section.
In another embodiment shown in FIG. 5, to be described, the slidable guiding means comprises a cylindrical guide surface 44' arranged eccentrically with respect to the main bearing section 12, and a pair of guide surfaces 40 on the housing extending transversely to the webs and operatively engaging the cylindrical guide surface 44'.
The degree of eccentricity of the cylindrical guide surface or the axis of the slide block journal, as the case may be, may be adjustable or fixed. If it is merely desired to alter the duration of the different parts of the combustion cycle, then a fixed eccentricity is sufficient, as shown. However, by permitting the eccentricity to be varied, as shown in FIG. 6, to be described, it is possible to construct an engine with variable compression ratio. This has application, for example, in a diesel engine in which cold starting problems can be mitigated by altering the compression ratio.